#include <fstream>
#include <iostream>
#include <cmath>

using namespace std;
int n;
typedef struct {int x, y;} punct;
long double dist(punct A, punct B)
{
    return sqrt((B.x-A.x)*(B.x-A.x)+ (B.y - A.y) * (B.y - A.y) );
}
long double arietr(punct A, punct B, punct C)
{
  return abs((B.x-A.x)*(C.y-A.y) - (C.x-A.x) * (B.y-A.y)) /2 ;
}
long double arie (punct p[100002])
{
    int i;
    long double ar=0;
    for (i=2; i<n; ++i)
{
     ar+=arietr(p[1],p[i],p[i+1]);
     cout << arietr(p[1],p[i],p[i+1]) << "\n";
}
    return ar;
}
int main()
{
   ifstream f("aria.in");
   ofstream g("aria.out");
   punct A[100002];
   punct q, w, e;
   int i;
   f >> n;
   for (i=1; i<=n; ++i)
   {
       f >> A[i].x >> A[i].y;
   }
   g << arie(A) << "\n";
    return 0;
}